1. Field of the Invention
The invention relates to a corrosion-resistant tension member, particularly a tendon for prestressed concrete.
2. Description of the Background Art
In the construction of buildings with prestressed concrete, bonded or unbonded prestressing is commonly known. With bonded prestressing, the tendons are located longitudinally movable within the concrete cross section and, after tensioning against the hardened concrete, are bonded to the surrounding concrete by injecting cement paste. With unbonded prestressing, the tendons are most often located outside of the concrete cross section, however, they are supported against a structure; in this way, they can be inspected, re-tightened, and if necessary replaced at any time.
With tension members of this kind, so-called monostrands are frequently used as tension elements, that is, strands made of seven steel wires, each being enclosed by a plastic sheath, for example, polyethylene, that is applied by extrusion, to protect against corrosion, and which are embedded in a corrosion-protection substance, for example, grease, which fills wedges between the steel wires and a ring space between the strand and the sheath; also known are strands that are enclosed by two sheaths of this kind, for reinforced protection against corrosion.
The anchorage of the strands at the ends of the tendons usually includes anchoring discs made of steel, with conical, and subsequently cylindrical bores in the number of strands, through which these are threaded and in which they are anchored with multiple-part ring wedges. To anchor the strands, it is, however, necessary to remove the sheaths from the strands in the area of the anchorage, so that the anchorage wedges can directly grip the bare strand.
For reasons of corrosion protection, the hollow spaces in the anchorage areas, where the sheaths were removed from the strands, must be filled with a material, for example, grease, to insure protection against corrosion. When the hollow spaces between the individual strands in the areas of the tendons in between the anchorings are filled in at their ends with a hardened material, for example, mortar, to safeguard against corrosion, it is necessary to tightly delimit the anchorage areas that are to be filled with corrosion-resistant materials from those areas.
To separate the anchorage areas, which are to be filled in with corrosion-resistant materials, of a tension member from the free areas, it is known to use sealing elements made of an elastic material around the individual sheathed strands, the sealing elements being brought to a transverse extension by a surface pressure in an axial direction of the tendon, to tightly seal off the individual strands and an interior wall of the outer sheathing. Seals such as these, designed somewhat like a compression gland, are known from EP 0 323 285 B2 and WO 01/20098 A1. In there, to activate the seals, pressure is applied to the sealing elements embedded between pressure plates by bolts that can be actuated from the exposed side of the anchor plate. This type of activation of the seals, however, necessitates a lot of effort.
If signs of corrosions appear on the individual strands despite all safety measures, their tension must be decreased to replace them. To do this, the bolts, which compress the seals between the pressure plates, must be loosened prior to loosening the ring wedges of the strands. Due to deformations that took place, the sealing parts frequently cannot be returned to their original position without additional expenditure of energy. Thus, the entire anchor plate has to be dismantled in order to replace individual strands to avoid the risk of damaging the deformed sealing elements and/or the sheaths of the strands when the strands are pulled.
Unbonded tendons, which traditionally have been used basically as external tendons, that is, tendons guided outside the concrete cross section, are increasingly also used as internal tendons, that is, tendons guided inside the concrete cross section. As tendons arranged inside the concrete cross-section they have an advantage from a static view point, namely, with regard to a lever arm of internal forces that can be utilized. Moreover, the tension can be controlled by re-tightening, which is not possible with bonded pre-loading. Lastly, this type of tendons allows replacement of individual tension members as well as the entire bundle.
Particularly advantageous compared to external tendons is the fact that the tendons are embedded in concrete so that reversing forces at reversing points can be absorbed without taking any particular measures. For this purpose, strands with reinforced sheaths or twice-extruded strands are also frequently used.